Black dots in render with many lamps
By default cycles samples one lamp at a time. The black spots likely correspond to samples where cycles was attempting to sample lights which are doing nothing for actually illuminating the visible objects.

You could try disabling Multiple Importance Sampling on lamps/materials you know won't contribute much to illuminating the scene, and/or play with the Light Sampling Threshold in Properties > Render settings > Sampling.

However, if that doesn't cut it, then you can use the Branched Path Integrator, which allows for sampling all lights at once:



Note that it works a bit differently from the default integrator, so try playing around with the settings to see what works.